The present disclosure relates generally to electrochemical cells, and particularly to venting gasses that result from operation of electrochemical cells.
Electrochemical cells are energy conversion devices, usually classified as either electrolysis cells or fuel cells. A proton exchange membrane electrolysis cell can function as a hydrogen generator by electrolytically decomposing water to produce hydrogen and oxygen gas, and can function as a fuel cell by electrochemically reacting hydrogen with oxygen to generate electricity. Referring to FIG. 1, which is a partial section of a typical anode feed electrolysis cell 100, process water 102 is fed into cell 100 on the side of an oxygen electrode (anode) 116 to form oxygen gas 104, electrons, and hydrogen ions (protons) 106. The reaction is facilitated by the positive terminal of a power source 120 electrically connected to anode 116 and the negative terminal of power source 120 connected to a hydrogen electrode (cathode) 114. The oxygen gas 104 and a portion of the process water 108 exits the cell 100, while protons 106 and water 110 migrate across a proton exchange membrane 118 to cathode 114 where hydrogen gas 112 is produced.
Another typical water electrolysis cell using the same configuration as is shown in FIG. 1 is a cathode feed cell, wherein process water is fed on the side of the hydrogen electrode. A portion of the water migrates from the cathode across the membrane to the anode where hydrogen ions and oxygen gas are formed due to the reaction facilitated by connection with a power source across the anode and cathode. A portion of the process water exits the cell at the cathode side without passing through the membrane.
A typical fuel cell uses the same general configuration as is shown in FIG. 1. Hydrogen, from hydrogen gas, methanol, or other hydrogen source, is introduced to the hydrogen electrode (the anode in fuel cells), while oxygen, or an oxygen-containing gas such as air, is introduced to the oxygen electrode (the cathode in fuel cells). Water can also be introduced with the feed gas. Hydrogen electrochemically reacts at the anode to produce protons and electrons, wherein the electrons flow from the anode through an electrically connected external load, and the protons migrate through the membrane to the cathode. At the cathode, the protons and electrons react with oxygen to form water, which additionally includes any feed water that is dragged through the membrane to the cathode. The electrical potential across the anode and the cathode can be exploited to power an external load.
In other embodiments, one or more electrochemical cells can be used within a system to both electrolyze water to produce hydrogen and oxygen, and to produce electricity by converting hydrogen and oxygen back into water as needed. Such systems are commonly referred to as regenerative fuel cell systems.
Electrochemical cell systems typically include a number of individual cells arranged in a stack, with the working fluids directed through the cells via input and output conduits or ports formed within the stack structure. The cells within the stack are sequentially arranged, each including a cathode, a proton exchange membrane, and an anode. The cathode and anode may be separate layers or may be integrally arranged with the membrane. Each cathode/membrane/anode assembly (hereinafter “membrane-electrode-assembly”, or “MEA”) typically has a first flow field in fluid communication with the cathode and a second flow field in fluid communication with the anode. The MEA may furthermore be supported on both sides by screen packs or bipolar plates that are disposed within, or that alternatively define, the flow fields. Screen packs or bipolar plates may facilitate fluid movement to and from the MEA, membrane hydration, and may also provide mechanical support for the MEA.
In order to maintain intimate contact between cell components under a variety of operational conditions and over long time periods, uniform compression may be applied to the cell components. Pressure pads or other compression means are often employed to provide even compressive force from within the electrochemical cell.
As a result of normal operation of the cell 100, at least one of the hydrogen gas 112 and the oxygen gas 104 will include moisture or water vapor 110, 108. A gas dryer is often used to remove the moisture 110, 108 from at least one of the hydrogen gas 112 and the oxygen gas 104. Use of the gas dryer will produce two gas streams: one dried gas stream that is provided as the desired end product and one moist gas stream that is vented to transport the moisture removed from the dried gas stream.
Electrochemical cell systems adapted for outdoor use, where temperatures may fall below the freezing point of water, can incorporate a number of arrangements to prevent freezing of a vent stack used to transport the moist gas stream. Examples of such arrangements include electrical heating tapes disposed surrounding the vent stack of the moist gas, and/or additional dried carrier gases injected into the vent stack to increase a flow rate and thereby effect a volumetric dilution of the moisture. Electrical heating tapes represent additional components of the cell system that may require maintenance and service, and therefore reduce an overall reliability of the electrochemical cell system. When used in a potential presence of hydrogen, electrical heating tapes must be rated for an explosive environment, which adds to a cost and complexity of the system. Furthermore, electricity consumed by electrical heating tapes lowers an overall net efficiency of the electrochemical cell. Dried carrier gases require additional components, such as valves and solenoids for example, to be incorporated into the electrochemical cell system and can result in a similar reduction in overall system reliability. Further, dried carrier gases represent an additional operating cost of the cell system as a consumable material. Accordingly, a need exists for an improved gas venting arrangement that overcomes these drawbacks.